Early colonizing dental plaque bacteria such as Streptococcus gordonii, form the substratum to which later-colonizing microbial species, including potential pathogens, attach in the process of mature plaque development. The S. gordonii single glucosyltransferase (GTF) enzyme hydrolyzes sucrose to form glucan polymers that can facilitate bacterial accumulation on surfaces. The level of GTF activity affects the ability of S. gordonii cells to colonize surfaces in vitro and therefore, may affect their ability to establish in ecological niches in vivo. Environmental and growth conditions are known to affect S. gordonii GTF activity. Expression of the GTF structural gene, gtfG, is positively regulated by the upstream determinant, rgg. S. gordonii undergo a reversible phase variation between high and low levels of GTF activity. Although the molecular basis for GTF phase variation is not known, nucleotide changes or rearrangements in rgg or gtfG are not necessary; thus distally- located DNA is implicated in the control of gtfG expression. The working hypothesis of the proposed studies is that regulation of GTF activity in S. gordonii involves complex molecular interactions that may have ecological implications for survival of S. gordonii in the oral cavity. The goal of the proposed studies, to identify environmental signals and genetic elements involved in control of S. gordonii GTF activity, will be pursued in four specific aims: 1) to characterize the molecular mechanism of rgg regulation of gtfG through structural and genetic studies; 2) to use reporter genes fusions to identify distally-located S. gordonii genes that affect rgg and gtfG expression; 3) to identify environmental conditions that affect expression of gtfG and its regulatory gene(s); 4) to use random arbitrarily primed PCR to identify additional S. gordonii genes that show altered levels of expression in response to genetic or environmental factors that affect GTF activity. It is hoped that these studies will provide an understanding of the molecular interactions and conditions that favor or disfavor glucan production by the commensal organism S. gordonii. In the long term, such information may provide insights into factors which control, and possibly select for, a healthy oral flora and potentially be implemented in the development of biologically based therapeutic regimens.